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Biomedical subjects

H Grundfest

Publications and source records attributed to H Grundfest.

At least 19 recordsLinked to original sources
PubMed1977

Regional differences in K channels of abdominal and circumesophageal segments of the crayfish medial giant axon.

Intracellular recordings reveal that the membrane of the circumesophageal region of the medial giant axon of crayfish responds to replacement of C1 with propionate differently from that of the abdominal region of the same axon. The connective hyperpolarizes in the propionate saline, whereas the abdominal region undergoes the transient depolarization that is expected when a permeant anion (Cl) is replaced with an impermeant one (propionate). The hyperpolarization of the connectives is accompanied by an increased conductance, a decreased length constant, and an increase in threshold current for intracellular stimulation. These effects are specific for the connectives and for propionate. They do not occur on replacing Cl with other large anions, isethionate, methane sulfonate, or glucuronate. The effects of propionate are independent of Na or Ca and result from an increased K conductance. The hyperpolarization induced by propionate is increased in a K-free saline, where the resting potential (EM) is considerably positive to the emf of the K battery (EK). It is abolished in elevated Ko when EM=EK.

Action Potentials↗
PubMed1976

Temperature dependence of the four ionic processes of spike electrogenesis in eel electroplaques.

Spike electrogenesis of eel electroplaques involves four ionic processes which are controlled by the membrane potential. A threshold depolarization causes normally closed Na permselective channels to open (Na-activation) and normally open K channels to close (K-inactivation). The Na channels then close (Na-inactivation), and as the spike is terminated, the K channels reopen (K-reactivation). The temperature dependence of these four processes has been examined in the present work. Opening of the Na channels and closure and reopening of the K-channels are either effectively instantaneous or are relatively independent of temperature in the range of at least 5 degrees to 22 degrees. Closure of the Na-channels has a Q10 (increase in rate of reaction for each 10 degrees increase in temperature) of about 9, and activation energy (Ea) of this reaction is about 31.5 kcal/mole (132 kJ/mole).

Action Potentials↗
PubMed1975

Sites of action of D2O in intact and skinned crayfish muscle fibers.

The effect on tension development of replacing 90% of the H2O of the bathing saline with D2O was studied on intact single fibers, and on skinned fibers before and after the latter were treated so as to eliminate Ca-accumulation by the sarcoplasmic reticulum (SR). Excitation-contraction coupling (ECC) of intact fibers is not abolished, but is depressed by D2O so that higher depolarizations are required to elicit a given tension. The reduction in tension at a given level of depolarization is not due to inhibition of the contractile system. The latter showed an enhanced Ca sensitivity; that is, skinned fibers respond to Ca concentrations that are 1-2 orders of magnitude smaller in D2O than in H2O saline. When bathed in D2O saline, intact fibers or skinned fibers with functional SR can still accumulate and release Ca in sufficient quantities to allow repeated induction of maximum tensions. Relaxation is slowed in all three types of preparation, perhaps because of an increased affinity of troponin to Ca in D2O salines.

Animals↗
PubMed1974

Calcium binding and tension development in detergent-treated muscle fibers.

The nonionic detergent Brij 58 eliminates irreversibly the capability of the sarcoplasmic reticulum (SR) of skinned crayfish muscle fibers to sequester Ca and to release it under appropriate stimulation. In contrast to deoxycholate (DOC) which causes an irreversible diminution of tension as well, Brij 58 does not affect the contractile proteins. Comparison of the time-course of tension development before and after Brij treatment demonstrates that Ca is accessible to the contractile proteins more rapidly after the SR is destroyed but, nevertheless, much more slowly than is predicted for free diffusion of Ca in the myoplasm. Slowing apparently results because of the presence of ca 1 mmol/kg fiber of myoplasmic Ca-binding sites that remain after Ca uptake of the SR is eliminated. A theoretical model is presented which allows for the effects of binding sites and of an unstirred layer in the vicinity of the fiber on Ca diffusion into the myoplasm.

Animals↗
PubMed1972

Regulation of tension in the skinned crayfish muscle fiber. II. Role of calcium.

Tension outputs were measured in skinned crayfish muscle fibers exposed to solutions variously buffered for both Mg-adenosine triphosphate (ATP) and Ca. Two types of data are shown, relating tension and substrate concentration with different levels of Ca present, or tension and calcium concentration at different levels of substrate. The data are fitted by curves calculated from a general equation for substrate inhibition. The equation is based on the schema that both tension and relaxation are induced by the substrate and that the relaxing effect of excess substrate is repressed by calcium. The physiological findings of the present work are similar to data obtained by others on biochemical model systems of the contractile proteins.

Actomyosin↗
PubMed1972

Membrane calcium activation in excitation-contraction coupling.

Depolarization thresholds for eliciting tension and Ca electrogenesis have been compared in isolated crayfish muscle fibers. Just-detectable tensions and Ca spikes induced after treatment with procaine were elicited with intracellularly applied depolarizing currents of fixed duration. Both thresholds were found to increase in a similar manner in fibers exposed to increased concentrations of Ca in the bathing solution or addition of other divalent cations (Mg, Mn, Ni). However, antagonistic effects between divalent cations were also demonstrated. Substitution of increasing amounts of NaSCN for NaCl in the standard saline produced a progressive decrease in both thresholds. The correlation in the change in thresholds for the two processes supports the hypothesis that a change in membrane Ca conductance is an integral step in excitation-contraction coupling.

Animals↗
PubMed1972

Effect of black widow spider venom on the lobster neuromuscular junctions.

The effect of black widow spider venom (BWSV) on the junctions of the lobster nerve-muscle preparation was studied by intracellular recordings. After application of BWSV both excitatory and inhibitory postsynaptic potentials (epsp and ipsp) were augmented then suppressed. The frequency of miniature potentials was markedly increased by BWSV. Summated postsynaptic conductance changes appeared to be responsible for the membrane depolarization and the decrease in effective membrane resistance seen in the early stages of the venom action. In the later stages both excitatory and inhibitory "giant miniature potentials" were evoked. No discernible changes were found in the reversal potential of the epsp and ipsp and in the sensitivity of the postsynaptic membrane. The results indicate that BWSV has a presynaptic action at crustacean neuromuscular junctions.

Animals↗
PubMed1971

Synaptic electrogenesis in eel electroplaques.

Whether evoked by neural or by chemical stimulation, the synaptic membrane of eel electroplaques contributes a depolarizing electrogenesis that is due to an increased conductance for Na and K. The reversal potential (E(S)) is the same for the two modes of synaptic activation. It is inside-positive by about 30-60 mv, or about midway between the emf's of the ionic batteries for Na (E(Na)) and K(E(K)). The total conductance contributed by synaptic activity (G(S)) varied over a fivefold range, but the individual ionic branches, G(SSNa), and G(SSK), change nearly equally so that the ratio G(SSNa):G(SSK) is near unity. G(SSK) increases independently of the presence or absence of Na in the bathing medium, and independently of the presence or absence of the electrically excitable G(K) channels. When activated, the synaptic membrane appears to be slightly permeable to Ca and Mg. When the membrane is depolarized into inside positivity the conductance of the synaptic components decreases and approaches zero for large inside-positive values. Thus, the synaptic components become electrically excitable when the potential across the membrane becomes inside-positive, responding as do the nonsynaptic components, with depolarizing inactivation.

Animals↗
PubMed1971

Regulation of tension in the skinned crayfish muscle fiber. I. Contraction and relaxation in the absence of Ca (pCa is greater than 9).

In isolated skinned crayfish muscle fibers bathed in solutions that were buffered to be virtually free of Ca(2+) (pCa 8-10) the substrate for both contraction and relaxation is the MgNTP complex. Tension increased up to 50% of the maximum capability of the fiber as the substrate MgATP increased to an optimum (pMgATP = 5.5). Relaxation was induced by further increases in MgATP. Similar bell-shaped curves of tension vs. pMgNTP were obtained with UTP and ITP, but optimum pMgUTP was about 4.5 and optimum pMgITP was about 2.6. The relation between equilibrium tension and pMgNTP is described by an equation analogous to that for the kinetics of enzymes regulated by substrate inhibition.

Adenosine Triphosphate↗
PubMed1971

The hyperpolarization of frog skeletal muscle fibres induced by removing potassium from the bathing medium.

1. The time course of changes in resting potential after removing K(0) was studied in twenty-four single fibres and in 136 fibres from small bundle (two to four fibres) preparations of frog semitendinosus muscles.2. The initial resting potentials in the control saline ranged between -88 and -98 mV. The potentials returned to nearly the initial values when control conditions were reinstated after 3-8 hr of experimentation. All the fibres twitched at the end of the experiment.3. Only about one third of the fibres hyperpolarized for any length of time on exposure to a K-free saline at room temperature (20-28 degrees C). The hyperpolarization was reversed to depolarization after a variable delay. The resting potential could fall to -50 or -40 mV.4. The remainder of the fibres depolarized with little or no prior hyperpolarization.5. Both patterns of response could be replicated in the different fibres.6. Hyperpolarization induced by K-free solution was reduced or abolished on cooling to ca 10 degrees C; on substitution of Tris or Li for Na; and upon inhibition of the Na pump with DNP (0.025-0.2 mM) or ouabain (0.05 mM). The latter agent was not as effective as the other conditions.7. Only small, slowly developing depolarization occurred when Na was replaced with Tris or Li.8. The various effects in K-free solutions were reversed on returning to the control conditions.9. It is suggested that removal of K(0) itself has little or no direct effect on the resting potential and that the initial hyperpolarization is due to the pumped efflux of Na without a compensatory influx of K. Block of the pump electrogenesis is manifested by depolarization of the fibres as K(1) is depleted and Na(1) increased.10. The Na pump appears to be dependent upon the nutritional status of the frogs and variations of the latter probably cause the different responses of fibres to removal of K(0).

Animals↗
PubMed1970

Potassium inactivation and impedance changes during spike electrogenesis in eel electroplaques.

Various degrees of pharmacological K inactivation were induced by Cs or Ba in isolated single electroplaques of the electric eel. The resulting changes in K conductance give rise to very different steady-state current-voltage characteristics. They also induce differences in ion dynamics during spike electrogenesis. The dynamic changes were studied by AC bridge methods, registering the changes in impedance in synchrony with the neurally or directly evoked spikes. While spike electrogenesis was virtually unaffected by addition of Cs or Ba, the patterns of impedance changes were very different. The various patterns are accounted for by the changes in the respective current-voltage characteristics. The data constitute new evidence for regarding the electrically excitable component of the reactive membrane as a heterogeneous electrochemical system with separate and independently reactive channels that in the electroplaques are permselective for Na and K, respectively.

Animals↗
PubMed1970

Effects of caffeine on crayfish muscle fibers. I. Activation of contraction and induction of Ca spike electrogenesis.

Contractions are evoked in single muscle fibers of crayfish by intracellular as well as extracellular applications of caffeine. Responses to external applications in concentrations above 2 mM could be induced indefinitely. With concentrations above 5 mM the caffeine-induced responses were highly repeatable. Tensions were transient even when the caffeine remained in the bath. There was no change in resting potential, but during the contraction the effective resistance decreased about 10%. A number of factors (change in pH, Ca, K, and Cl) modified the responses. The time course of the tension was greatly prolonged when the transverse tubular system (TTS) was s swollen and was again shortened when the TTS was caused to shrink. An increased permeability to Ca induced by caffeine was evidenced by the transformation of the normally graded electrical responses to Ca spikes, which are insensitive to tetrodotoxin. The overshoot is a function of both external Ca and caffeine. A 10-fold change in Ca changed the overshoot by 19 mv in the presence of 10 mM caffeine and by 29 mv in 80 mM caffeine. The role of the increased permeability to Ca for caffeine-induced contractions will be analyzed in the accompanying paper.

Animals↗
PubMed1970

Effects of caffeine on crayfish muscle fibers. II. Refractoriness and factors influencing recovery (repriming) of contractile responses.

When caffeine evokes a contraction, and only then, crayfish muscle fibers become refractory to a second challenge with caffeine for up to 20 min in the standard saline (5 mM K(o)). However, the fibers still respond with contraction to an increase in K(o), though with diminished tension. Addition of Mn slows recovery, but the latter is greatly accelerated during exposure of the fiber to high K(o), or after a brief challenge with high K(o). Neither the depolarization induced by the K, nor the repolarization after its removal accounts for the acceleration, which occurs only if the challenge with K had itself activated the contractile system; acceleration is blocked when contractile responses to K are blocked by reducing the Ca in the bath or by adding Mn. Recovery is accelerated by redistribution of intracellular Cl and by trains of intracellularly applied depolarizing pulses, but not by hyperpolarization. The findings indicate that two sources of Ca can be mobilized to activate the contractile system. Caffeine mobilizes principally the Ca store of the SR. Depolarizations that are induced by high K(o), by transient efflux of Cl, or by intracellularly applied currents mobilize another source of Ca which is strongly dependent upon the entry of Ca from the bathing medium. The sequestering mechanism of the SR apparently can utilize this second source of Ca to replenish its own store so as to accelerate recovery of responsiveness to a new challenge with caffeine.

Animals↗
PubMed1970

Desensitization of gamma aminobutyric acid (GABA) receptors in muscle fibers of the crab Cancer borealis.

Carcinus muscle fibers respond to gamma-aminobutyric acid (GABA) with a conductance increase that subsides rather rapidly. In the larger fibers which have low input resistance the decrease may disappear within 2 min. The inhibition of the excitatory postsynaptic potentials (EPSP's) by GABA nevertheless persists as long as the drug is applied. The subsidence of the increased conductance indicates that the membrane of the inhibitory synapses has become desensitized to GABA. The persistence of inhibition of the EPSP's appears to be due to an action of the drug on the presynaptic terminals of the excitatory axons which reduces or blocks the secretory activity that releases the excitatory transmitter.

Aminobutyrates↗
PubMed1969

Ionic permeability of the inhibitory postsynaptic membrane of lobster muscle fibers.

Reversal potentials (E(IPSP)) of the inhibitory postsynaptic potential and the membrane resting potentials (E(M)) of lobster muscle fibers were determined with intracellular recording under a variety of ionic conditions. E(IPSP) is solely dependent on the electromotive force of anionic batteries; i.e., on the electrochemical gradient for a "mobile" fraction of intracellular Cl (Cl(i)) which is considerably smaller than the total intracellular Cl. The active inhibitory membrane is more permeable to certain "foreign" anions in the order NO(3) > SCN > Br > Cl. The membrane is impermeable to BrO(s), isethionate, and methylsulfate, but is slightly permeable to acetate and propionate. The level of Cl(i) appears to be determined in part by some active (pump?) process and most of the anions studied appear to interfere with the steady-state level of Cl(i).

Animals↗